1. Field of the Invention
The present invention relates to a physical quantity detecting device for detecting predetermined physical quantities and for generating digital data in accordance with the physical quantities.
2. Related Art
Physical quantity detecting devices for detecting physical quantities such as the rotational position and rotational speed of an object of interest as those disclosed in Japanese unexamined patent publication No. H3-125514 and No. H3-220814 have been known. They are configured to detect a transition of a physical quantity of an object of interest such in a manner wherein two pulse signals the phase difference between which changes with a change in the physical quantity are generated using a pair of pulse signal generators each comprising a detection element whose characteristics change with a change in the physical quantity, an oscillator as a physical quantity-to-time base conversion circuit for a pulse signal at a frequency corresponding to the characteristics of the detection element, and a frequency divider as a time base amplifier circuit for amplifying the pulse output by the oscillator by dividing the frequency thereof; the phase difference between the two pulse signals thus generated is converted into digital data using a pulse phase difference encoding circuit as a time A-D converter; and an increase or decrease in this digital data is detected.
This type of physical quantity detecting device digitally processes a physical quantity to be detected by converting it into a phase difference between pulse signals. Therefore, the use of a magnetic resistance element as a rotation sensor makes it possible to detect a very small change in rotation which cannot be detected by conventional analog sensors such as magnetic pick-up sensors. Further, such a rotation sensor will exhibit high endurance against environmental conditions because the signal processing circuit is constituted by a digital circuit.
In order to improve the accuracy in detecting a physical quantity with this type of physical quantity detecting device, the sampling cycle (i.e., operating cycle) of the pulse phase difference encoding circuit for converting the phase difference between the pulse signals frequency-divided by the frequency divider into digital data must be short to perform sampling at short intervals and the degree of signal amplification (i.e., division ratio) at the frequency divider for amplifying the oscillation signal from the oscillator by frequency-dividing the same must be high.
However, in order to make the sampling cycle of the pulse phase difference encoding circuit short, the degree of signal amplification of the frequency divider must be low. Conversely, in order to raise the degree of signal amplification at the frequency divider, the sampling cycle of the pulse phase difference encoding circuit must be shortened.
In consequence, the detecting accuracy of such a physical quantity detecting device cannot be improved by simply increasing or decreasing the sampling cycle of the pulse phase difference encoding circuit and the degree of signal amplification at the frequency divider.